The failure to induce broadly neutralizing antibodies against primary isolates of human immunodeficiency virus (HIV) poses a major impediment to the development of effective vaccines against AIDS. Many approaches have been explored with the goal to enhance immune responses against the HIV-1 envelope proteins. So far, success has been incremental. It is likely that a combination of better understanding of the structure and function of the envelope antigen, together with rational design and systematic examination of novel immunogens, will be necessary to achieve this goal. Our laboratory has been interested in the role of N-linked glycans on the functional and immunogenic properties of HIV-1 envelope proteins. As summarized in the Preliminary Results, we have shown that removal of a single N-linked glycan in HIV-1 gp120 resulted in increased sensitivity to broadly neutralizing monoclonal antibodies as well as the ability to mediate CD4-independent viral infection and to induce cross-reactive neutralizing antibody responses in macaques. These observations led us to propose the following hypotheses: (1) Structural changes induced by specific glycan modifications leading to the acquisition of CD4-independent phenotype and the exposure of conserved epitopes provide in part the basis for the rational design of envelope antigens with enhanced immunogenicity;and (2) incorporation of such Env antigens in an optimized immunization regimen will help generate cross-neutralizing antibody responses that contribute to the protection against HIV-1 infection and disease. In this application, we propose to further define the nature of the cross-reactive neutralizing antibody response elicited by the mutant Env and to examine the basis and the general applicability of this observation. Env from mutant CD4-indepedent viruses representing multiple subtype B and C isolates will be used in a novel immunization approach designed to Preferentially Expand Antibodies to Common Epitopes (PEACE). Protective efficacy of this approach will be evaluated in macaques against SHIV challenge. Information from these studies will provide a rational basis for the design of effective vaccines against HIV/AIDS. The specific aims of this proposal are: 1. To further define the nature of the cross-reactive neutralizing antibody responses elicited by glycan modified Env that impart the CD4-independent phenotype 2. To examine the potential correlation between the ability of mutant Env protein to mediate CD4- independent infection and its ability to induce broadly cross-reactive neutralizing antibody responses 3. To optimize a sequential immunization approach designed to Preferentially Expand Antibodies to Common Epitopes (PEACE) and to evaluate its protective efficacy in macaque models